In recent years, wireless LAN (Local Area Network) communications have been used not only in offices but also in wide areas such as homes and outdoor. Along with widespread use of the wireless LAN, the amount of communications in limited frequency resources increases, whereby communication interference with each other has become a serious problem.
In IEEE802.11 which is one of the wireless LAN standards, an access control method of carrier sense multiple access with collision avoidance (CSMA/CA) is used (see Patent Document 1 and Non-Patent Document 1). In this access control method, when each communication device (wireless communication device) attempts to transmit a wireless frame, it checks whether or not neighboring communication devices are sending radio waves before starting transmission of a wireless frame.
Further, in the aforementioned access control method, in order to avoid collision of wireless frames, when any neighboring communication device is sending a radio wave, a communication device waits for a certain period of time (back-off time), and then checks again whether or not a radio wave is sent, and if any neighboring communication device is not sending a radio wave, the communication device transmits a radio wave by itself after a random time elapses. The communication device uses carrier sense to check whether or not neighboring communication devices are not sending radio waves.
A communication device checks a use state of a wireless channel by using carrier sense. When detecting preamble of a signal (signal for establishing synchronization) conforming to the IEEE802.11 standard, the communication device receives the signal, so that the state becomes a wireless channel in-use state (busy). When the communication device does not detect preamble of a signal conforming to the IEEE802.11 standard and detects an electric power level higher than a preset carrier sense threshold, the communication device determines that the state is busy, and stands by for transmission. Meanwhile, when detecting an electric power level lower than a carrier sense threshold, the communication device determines that the wireless channel is not used (idle).
Patent Document 2 discloses a technology as described below. In a communication system in which two base stations are able to perform transmission simultaneously, when data to be transmitted is generated, each base station transmits transmission notice information in a frame immediately before starting transmission of the data. A mobile station measures an interference level based on the received transmission notice information, determines a request rate from the measured interference level, and notifies the based station of it. The base state transmits data by using the request rate as a downstream data rate. Then, the based state checks whether or not the data is being transmitted, and when it is being transmitted, transmits in-transmission information. The mobile station measures the interference level based on the received in-transmission information, determines a request rate, and notifies the base station of it.
Patent Document 3 discloses a communication device in which interference with other communications is suppressed, and data transmission efficiency is improved. Specifically, a communication device includes a data transmission and reception unit that wirelessly transmits a plurality of test packets, a signal sensing unit that senses electric power of a spatial radio wave signal on a frequency channel that is the same as the that of the test packets and outputs sample data of the spatial radio wave signal, a calculation processing unit that converts the sample data into time-series sample data that is data in which sample data is plotted in a time-series manner, and a collision detection unit that determines packet collision due to interference between the test packets and other communications based on the time-series sample data.    Patent Document 1: WO2007/091738 A    Patent Document 2: JP 2005-244857 A    Patent Document 3: JP 2013-5097 A    Non-Patent Document 1: Wireless LAN Standard IEEE802.11    Non-Patent Document 2: ISO-IEC 7498-1    Non-Patent Document 3: Analysis of Open Source Drivers for IEEE 802.11    Non-Patent Document 4: Fwd FW Channel busy cycles, [searched on Aug. 5, 2014], Internet <URL: http://permalink.gmane.org/gmane.linux.drivers.ath9k.devel/9887>